1. Field of the Invention
The present invention relates to an analog buffer circuit such as a source follower circuit using a polycrystalline thin film transistor. The invention also relates to a signal driver circuit and an active matrix liquid crystal display device having the source follower circuit of the invention.
2. Description of the Related Art
Recently, a technology regarding a semiconductor device formed on an insulating substrate by using a thin film transistor (hereinafter referred to as a TFT) is rapidly developed. Further, as for a semiconductor device having a polycrystalline semiconductor film by making an amorphous semiconductor film into a polycrystalline semiconductor film by heating or laser irradiation, a pixel portion and a driver circuit can be integrated; therefore, miniaturization and realization of a narrower frame (to increase the ratio of the pixel portion to the display portion) of a semiconductor device are being aimed at.
Meanwhile, a demand and expectation for a large semiconductor device in 15 to 30 inches in size, especially for a liquid crystal display device are growing. Such a large liquid crystal display device is quite thin, light and flexible in design as compared with a CRT display device of the same size.
In such a large liquid crystal display device, as a signal line and a scan line become longer, a wiring resistance increases and a signal is delayed due to the increased load. Therefore, a line sequential drive is performed in which a signal is stored in a memory in a signal driver circuit once and then written to a signal line. In this case, an analog buffer circuit is required to be disposed next to the memory.
A thin film transistor can be used in this analog buffer circuit; however, characteristics of the thin film transistor vary. Therefore, a method for correcting the variation of the thin film transistors is suggested. For example, the difference between a power supply voltage and a difference between an input voltage and a threshold voltage of a transistor is stored in a capacitor, and in the case of output, the input voltage which cancels the threshold value is held by using the transistor (refer to Patent Document 1).
Furthermore, in a source follower circuit configured with polysilicon TFTs, one end of a capacitor is connected to the gate of a source follower transistor, a first analog switch is connected between the gate of the source follower transistor and a precharge power source, a second analog switch is connected between the other end of the capacitor and the source of the source follower transistor, and a third analog switch is connected between the other end of the capacitor and a signal source. Thus, a precharge operation is performed and an offset is canceled at a high precision (refer to Patent Document 2). Patent Document 2 discloses that an NMOS transistor and the source follower transistor are coupled in a cascode configuration, a voltage Vgd between the gate and drain of the source follower transistor is maintained almost constant during the precharge period and the output period, and a depletion mode transistor is used as this cascode connected NMOS transistor.
Patent Document 1: Japanese Patent Application Laid-open No.2000-194323
Patent Document 2: Japanese Patent Application Laid-open No.Hei 11-73165